1. Field of the Invention
The present invention relates to a toner for developing an electrostatic latent image in an electrophotographic, electrostatic recording or electrostatic printing process, and more particularly to capsule toners adapted for pressure fixing.
2. Description of the Prior Art
In various electrophotographic processes as already disclosed in the U.S. Pat. Nos. 2,297,691, 2,825,814, 3,220,324 and 3,220,831 and British Pat. Nos. 1165406 and 1165405, an image reproduction is obtained by forming an electric latent image by various methods generally on a photoconductive material, developing said latent image with a toner, if desired transferring thus obtained toner image onto a transfer material such as paper, and finally fixing said image by heating, pressure or solvent vapor.
Such electric latent image can be rendered visible by various methods.
For example, there are already known a magnetic brush method as disclosed in the U.S. Pat. No. 2,874,063, a cascade development method as disclosed in the U.S. Pat. No. 2,618,552, a powder cloud development method as disclosed in the U.S. Pat. No. 2,221,776, a touch-down development method as disclosed in the U.S. Pat. No. 2,895,847, a fur brush development method, a liquid development method etc. The toner employed in such development methods has been a finely divided material composed of a natural or synthetic resin in which dyes or pigments are dispersed. Also there is also known to add a third material to such toner for various purposes.
The developed toner image is transferred onto a transfer material such as paper, if desired and finally fixed thereon.
Such image fixation is already known to be achievable by heat melting said toner with a heater or a heat roller and thus adhering said toner to a substrate, by softening or dissolving the binder resin of said toner with an organic solvent to obtain such adhesion, or by adhering the toner to the substrate by pressure.
In general, each toner is specifically designed for one developing method and is not applicable to other developing methods. Particularly the toner designed for the widely employed heat fixing method with a heater is scarcely applicable to other fixing methods such as heat roller fixation, solvent fixation or pressure fixation. For this reason various toners have been developed for such fixing methods.
The pressure fixation of toner, as disclosed in the U.S. Pat. No. 3,269,626 and in the Japanese Patent Publication Sho No. 46-15876, has various advantages such as economy in energy, absence of pollution, possibility of immediate copying without waiting time after power supply to the copying machine, absence of danger of copy scorching, possibility of high-speed fixation and simpler structure of fixing device.
However this fixing method is also associated with various troubles such as insufficient fixability of toner, toner offsetting to the pressurizing roller or adhesion of paper to said roller, and various trials have therefore been made to improve the pressure fixability.
For example there are already proposed a pressure fixable toner comprising an aliphatic component and a thermoplastic resin as disclosed in the British Pat. No. 1210665, a pressure fixable encapsulated toner with a soft core material as disclosed in the U.S. Pat. Nos. 3,788,994, 3,893,932 and 3,974,078, and in the Japanese Patents Laid-Open Sho No. 49-17739 and Sho No. 52-108134, and a pressure fixable toner utilizing a block copolymer of a tenace polymer and a soft polymer as disclosed in the Japanese Patent Laid-Open Sho No. 48-75033.
It has however been unable to obtain a practically satisfactory pressure fixable toner providing a sufficient pressure fixability, free from image offsetting or adhesion of paper to the pressurizing roller, stable in developing and fixing performance after repeated use, free from sticking to the carrier, metal sleeves or photosensitive member and provided with a satisfactory shelf life without coagulation or caking during storage.
For example a pressure fixable toner composed of a soft material, though showing a relatively good fixation by pressure, is associated with various troubles such as difficulty of crushing in preparation of toner particles, tendency of causing offsetting to the pressurizing roller or sticking to the carrier or photosensitive member, and coagulation or caking during storage.
Also the conventionally known pressure fixable capsule toners have been unsatisfactory in the practical performance since a soft core material showing a satisfactory pressure fixability will gradually deposit on the pressurizing roller after repeated fixing operations to cause toner off-setting or paper adhesion to said roller, while the fixing performance becomes deteriorated if such drawback is avoided.
Also in a recently employed developing method utilizing a single-component developer which is composed solely of toner particles containing magnetic minute particles therein and is free from carrier particles, the binder resin for said toner is required to provide satisfactory dispersibility for and adhesion to said magnetic particles as well as high impact strength and flowability in toner, which are not easily rendered compatible with the pressure fixing performance.